Since the early 1980s, thousands of patients prone to irregular and sometimes life-threatening heart rhythms have had miniature heart monitors, particularly defibrillators and cardioverters, implanted in their bodies. These devices detect onset of abnormal heart rhythms and automatically apply corrective electrical therapy, specifically one or more bursts of electric current to the heart. When the bursts of electric current are properly sized and timed, they restore normal heart function without human intervention, sparing patients considerable discomfort and often saving their lives.
The typical defibrillator or cardioverter includes a set of electrical leads, which extend from a sealed housing into the wall of a heart after implantation. Within the housing are a battery for supplying power, monitoring circuitry for dctccting abnormal heart rhythms, and a capacitor for delivering bursts of electric current through the leads to the heart.
Flat capacitors generally include a stack of flat capacitor elements, with each element including a paper separator between two sheets of aluminum foil. One of the foils serves as the anode of the capacitor element, and the other serves as the cathode.
One or more of the capacitor elements are often die cut in a shape designed to conform to a capacitor case. The cutting results in undesired residual stresses, and in warpage of the capacitor element. Stacking a plurality of these types of capacitor elements may result in increased height to the assembly. Moreover, the foil strip used to produce the capacitor element may not have the desired flatness prior to processing. Undesired residual stress due to this factor may also result in warpage of the capacitor assembly, enough to add height to the assembly. Moreover, the foils are cut using high-precision dies which are not only expensive, but require repeated sharpening. Another problem that arises is that cutting the foils can produce burrs on the cut edges of the foils. When edge burrs on adjacent anode and cathode foils contact each other, a conductive path results that short circuits the capacitive element.
Each anode foil in the stack, and each cathode foil in the stack, is interconnected to the other anodes and cathodes respectively. The anodes and cathodes generally include tabs which are crimped or welded together. Connecting the anodes and cathodes in this way provides a total capacitance equal to the sum of the capacitances of all the capacitor elements. However, the anode and cathode interconnections cause designers to increase the size of the capacitor case to accommodate tabs or to remove a portion of the capacitive elements, which reduces total capacitance or increases the size of the capacitor.
Moreover, since defibrillators and cardioverters are typically implanted in the left region of the chest or in the abdomen, a smaller size device, which is still capable of delivering the required level of electrical energy, is desirable.
Accordingly, there is a need for capacitor structures and methods of manufacture which provide greater process control, less expensive manufacturing, provide for a design efficiently utilizing space within the capacitor case, and provide for a compact capacitor design capable of providing the required pulse of energy for use within the implantable device.